Infectious Mononucleosis Testing

The cardinal symptoms of infectious mononucleosis (IM) are the well-known triad of fever, pharyngitis, and peripheral lymphadenopathy, especially involving the posterior cervical nodes. About 5% of patients present with rash, especially if they are being treated with ampicillin for pharyngitis. Adults older than 40 years are less likely to present with lymphadenopathy and pharyngitis and more likely to have hepatitis, cholestasis, and hepatomegaly.

Monospot is a rapid latex agglutination test for the detection of IgM heterophile antibodies that are present in patients with infectious mononucleosis. Heterophile antibody recognizes cells from a different species, such as horse red blood cells. They do not recognize viral epitopes and appear to arise from the immunological chaos created by viral infection of B-lymphocytes. Heterophile antibodies usually appear within 1 week after infectious mononucleosis begins and peak during weeks 2 to 5. They are generally short-lived, but can persist at low levels for up to a year. About 10% of all patients with IM, especially children and older adults, have heterophile negative disease that must be confirmed by ordering an EBV antibody panel.

More than 70% of patients with IM have an absolute lymphocytosis with values peaking during the second and third weeks of illness. Total WBCs usually range between 12,000 and 18,000 cells/uL, but may occasionally exceed 30,000 cells/uL. Typically 60 to 70% of the white blood cells are lymphocytes and monocytes. From 10 to 30% of all circulating lymphocytes may be atypical. Older patients are less likely to have significant lymphocytosis and atypical lymphocytes.

The final diagnosis of IM depends upon the combination of clinical, hematologic and serologic findings. A practical diagnostic approach to IM is shown in the following table.

Atypical Lymph

Monospot

Diagnosis of IM

Further Tests

Absent

Negative

Not confirmed

None

Present

Positive

Confirmed

None

Absent

Positive

Inconclusive

EBV Antibody Panel

Present

Negative

Suspicious

Repeat Monospot in 2 wk or EBV Panel

Patients with IM may have a negative Monospot test even though atypical lymphocytes are increased, because serological changes often lag behind peripheral blood changes. Most heterophile negative patients with EBV are young children. In this situation, the more sensitive EBV antibody panel can be ordered or the Monospot test can be repeated in two weeks. Other diseases, which can cause an IM-like syndrome, such as CMV, should also be considered.More rarely, the Monospot test may be positive, but atypical lymphocytes are not increased. This combination of results usually indicates a remote IM infection.

Results are reported as negative or positive. Reference value is negative.